In vehicles comprising an automatic transmission, when a shift lever of the automatic transmission is switched from a neutral (N) range to a drive (D) range in order for the vehicle to advance, for example, the hydraulic clutch is engaged in order to transmit the rotation torque of the engine to the drive wheel. Since an oil pressure supply path for supplying oil pressure to the hydraulic clutch is at zero when the hydraulic clutch is released, a time lag occurs between the supply of oil pressure to the oil pressure supply path and the engagement of the hydraulic clutch. If the speed of oil pressure supply is increased in order to reduce this time lag, shock is generated due to the rapid engagement of the hydraulic clutch.
JP2001-336625A, published by the Japan Patent Office in 2001, discloses a method of temporarily raising the oil pressure inside the oil pressure supply path which is supplied to the hydraulic clutch at the stage of shift lever operation, or in other words performing so-called precharging. When precharging is performed, hydraulic fluid is filled into the oil pressure supply path prior to the engagement action of the hydraulic clutch, and since the time lag in the engagement operation of the hydraulic clutch is eliminated, the subsequent engagement action of the hydraulic clutch can be performed smoothly and swiftly.